Play is not neatly defined in terms of any single characteristic; instead, it involves a constellation of characteristics, which have to do with the motives or mental framework underlying the observed behavior. Three famous examples of attempts to characterize play are the following:

In his classic book Homo Ludens, the Dutch cultural historian Johan Huizinga (1955) summed up his elaborate definition of play as follows: "Play is a free activity standing quite consciously outside ‘ordinary’ life as being ‘not serious,’ but at the same time absorbing the player intensely and utterly. It is an activity connected with no material interest, and no profit can be gained by it. It proceeds within its own proper boundaries of time and space according to fixed rules and in an orderly manner."

In his influential essay, The Role of Play in Development, the Russian psychologist Lev Vygotsky (1978) characterized children’s play as activity that is (a) “desired” by the child, (b) “always involves an imaginary situation,” and (c) “always involves rules” (which are in the minds of the players and may or may not be laid down in advance).

In an often-referred-to article on play in the Handbook of Child Psychology, Kenneth Rubin and his colleagues (1983) characterized play as behavior that is (a) intrinsically motivated; (b) focused on means rather than ends; (c) distinct from exploratory behavior; (d) nonliteral (involves pretense), (e) free from externally imposed rules; and (f) actively (not just passively) engaged in by the players.

After analyzing these and other attempts to define play, Peter Gray (2009, 2013) concluded that essentially all of the descriptors of human play used by prominent play scholars can be boiled down to the five described below.

Five Most Agreed-Upon Characteristics of Human Play

Play, at least in humans, is not necessarily all-or-none, but can exist in matters of degree. An activity can be characterized as play, or described as playful, to the degree that it contains the characteristics listed here: Play is activity that is (1) self-chosen and self-directed; (2) intrinsically motivated; (3) guided by mental rules; (4) imaginative; and (5) conducted in an active, alert, but relatively non-stressed frame of mind. Here is an elaboration of each of these characteristics:

Play Is Self-Chosen and Self-Directed

Play, first and foremost, is what one wants to do, as opposed to what one feels obliged to do. Players choose not only to play, but how to play, and that is the meaning of the statement that play is self-directed. In social play (play involving more than one player), one person may emerge for a while as the leader, but only at the will of all the others. Anyone may propose rules, but the rules must be agreeable to all if the behavior is to remain in the realm of play for all. The most basic freedom in play is the freedom to quit. The freedom to quit ensures that all of the players are doing what they want to do, and it prevents leaders from enforcing rules that are not at least tacitly agreed to by the other players.

Play is intrinsically motivated—means are more valued than ends.

Play is activity that, from the conscious perspective of the player, is done for its own sake more than for some reward outside of the activity itself. In other words, it is behavior in which means are more valued than ends. When people are not playing, what they value most are the results of their actions. A person may scratch an itch to get rid of the itch, flee from a tiger to avoid getting eaten, or work at a boring job for money. If there were no itch, tiger, or paycheck, the person would not scratch, flee, or work. When people are not playing, they typically opt for the least effortful way of achieving their goal. In play, however, all this is reversed. In play, attention is focused on the means more than the ends, and players do not necessarily look for the easiest routes to achieving the ends.

Play often has goals, but the goals are experienced as part and parcel of the activity, not as the primary reason for the activity. Goals in play are subordinate to the means for achieving them. For example, constructive play (the playful building of something) is always directed toward the goal of creating the object that the players have in mind; but the primary objective in such play is the creation of the object, not the having of the object once it is built. Similarly, competitive play is directed toward the goal of scoring points and winning, but if the activity is truly play, then it is the process of scoring and winning that matters to the player, not some subsequent consequence of having scored and won.

Competition can turn play into non-play if rewards for winning extend beyond the game itself. “Players” who are motivated primarily by trophies, praise, or increased status outside of the game are not fully playing. Among nonhuman animals there is a clear distinction between contests (including ritualized battles of bluff as well as actual fights), which are aimed at achieving dominance, and play, in which strivings for dominance must be set aside (Bekoff & Byers, 1998). Human competitive games can be understood as blends of contest and play. The blend can veer more in one direction or the other, depending on the degree to which heightened out-of-game status or other extrinsic rewards are present for winning.
Superficially, the statement that play is activity done for its own sake may seem to contradict theories about play’s functions, which posit that play promotes long-term physical, social, emotional, and intellectual gains. The contradiction is resolved by appeal to the players’ conscious motives. To the degree that a person engages in an activity for its long-term benefits as opposed to its immediate enjoyment or attraction, the activity is not fully play.

Play is guided by mental rules, but the rules leave room for creativity.

Play is freely chosen activity, but not freeform activity. Play always has structure, and that structure derives from rules in the players’ minds. In social play, the rules must be shared, or at least partially shared, by all of the players. The rule-based nature of play is the characteristic that Vygotsky (1978) emphasized most strongly, as he built his argument that play is the means by which children learn to control their impulses and abide by socially agreed-upon rules.

The rule-based nature of play is an extension of the point made above about the prominence of means in play. The rules of play are the means. The rules are not like rules of physics, nor like biological instincts, which are automatically followed. Rather, they are mental concepts that often require conscious effort to keep in mind and to follow. The rules of play provide boundaries within which the actions must occur, but they do not precisely dictate each action; they always leave room for creativity. Activities that are precisely prescribed by rules are better referred to as rituals rather than as play.

Different types of play have different types of rules. A basic rule of constructive play, for example, is that you must work with the chosen medium in a manner aimed at producing or depicting some specific object or design that you have in mind. In sociodramatic play (the playful acting out of roles or scenes, as when children play “house” or pretend to be superheroes) the fundamental rule is that players must abide by their shared understanding of the roles that they are playing; they must stay in character. Even playful fighting and chasing, which may look wild to the observer, is constrained by rules. An always-present rule in children’s play fighting, for example, is that the players mimic some of the actions of serious fighting, but don’t really hurt the other person. They don’t hit with all their force (at least not if they are the stronger of the two); don’t kick, bite, or scratch. Because of its rule-based nature, play is always an exercise in self-restraint.

Play is imaginative.

Play always involves some degree of mental removal of oneself from the immediately present real world. This is the characteristic that Huizinga (1955) emphasized most strongly, as he built his argument that play provides the engine for cultural innovations. This is also the characteristic most strongly emphasized by researchers who focus on the role of play in the development of creativity and the ability to think in ways that go beyond the concrete here-and-now. As Vygotsky (1978) pointed out, the imaginative nature of play is, in a sense, the flip side of play’s rule-based nature. To the degree that play takes place in an imagined world, the players’ actions must be governed by rules that are in the minds of the players rather than by laws of nature or impulsive instincts.

Imagination, or fantasy, is most obvious in sociodramatic play, where the players create the characters and plot, but it is also present in other forms of human play. In rough and tumble play, the fight is a pretend one, not a real one. In constructive play, the players may say that they are building a castle from sand, but they know it is a pretend castle, not a real one. In formal games with explicit rules, the players must accept an already established fictional situation that provides the foundation for the rules. For example, in the real world bishops can move in any direction they choose, but in the fantasy world of chess they can move only on the diagonals.

Play is conducted in an alert, active, but relatively non-stressed frame of mind.

This final characteristic of play follows naturally from the other four. Because play involves conscious control of one’s own behavior, with attention to means and rules, it requires an active, alert mind. Players have to think actively about what they are doing. Yet, because play is not a response to external demands, and because the activity takes place in a fantasy world rather than the real world, and because the ends do not have immediate consequences in the real world, the person at play is relatively free from pressure or stress.

Play is not always accompanied by smiles and laughter, and mental tension may arise as players strive to perform well; but, as play is always self-chosen, so is any mental tension that accompanies it. If the tension becomes too great, reaching the level of distress, the player is free to quit or change the structure of the play at any time and thereby relieve the tension. If an activity becomes compulsive, so that the person continues at it despite a high degree of mental distress, then we are inclined to say that the activity is no longer play.

The mental state of play is what Mihalyi Csikszentmihalyi (1990) has called flow. Attention is attuned to the activity itself, and there is reduced consciousness of self and time. The mind is wrapped up in the ideas, rules, and actions of the game and relatively impervious to outside distractions. This state of mind has been shown, in many psychological research studies, to be ideal for creativity and the learning of new skills (see Gray, 2013).

How Play is Identified in Nonhuman Animals

The five-characteristic definition of play presented above depends, for its usefulness, on ways of knowing or inferring the motives and attitudes of the individuals involved. Researchers who study play in humans can ask their subjects about motives and attitudes and can also gain some understanding of them through introspection. However, those who study play in nonhuman animals (and in pre-verbal human infants) must rely entirely on non-verbal behavioral cues. A number of researchers have attempted to spell out the cues that distinguish play from non-play in animals.

Most of these attempts begin with the observation that playful actions in animals usually resemble serious actions in the same species, but are conducted in manner or in situations that make it clear that the actions are not serving their usual serious, or instrumental purposes. Consider, for example, the difference between a cat that is preying on a mouse and a cat that is playing at preying on a mouse. The former takes the quickest route for killing the mouse. The latter tries various ways of catching the mouse, not all very efficient, and lets the mouse go each time so it can try again. The preying cat is focused on the end; the playing cat is focused on the means. To the preying cat the mouse is food; to the playing cat it is a toy.

One often-quoted definition of play applicable to animals is that presented in summary form by Marc Bekoff and John Byers (1981) as follows: “Play is all locomotor activity performed postnatally that appears to have no obvious immediate benefits for the player, in which the motor patterns resembling those used in serious functional contexts may be used in modified forms. The motor acts constituting play have some or all of the following structural features: exaggeration of movements, repetition of motor acts, and fragmentation or disordering of sequences of motor acts.”

Robert Fagen (1981), in his now classic book Animal Play Behavior, described the distinctive characteristics of play as repetition, reversal, fragmentation, exaggeration, inhibition, and unpredictability—all of which contribute to the inference that the activity is not serving an immediately useful function, though it may be serving a delayed function relevant to the development of skills.

More recently, Gordon Burghardt (2011) has characterized play, for all animals (including humans), in terms of the following five criteria, which, he argued, must all be present if the behavior is to be called play:

“The performance of the behavior is not fully functional in the form or context in which it is expressed; that is, it includes elements, or is directed toward stimuli, that do not contribute to current survival.”

“The behavior is spontaneous, voluntary, intentional, pleasurable, rewarding, reinforcing, or autotelic (done for its own sake).” This characteristic, more than the others, seems to require an inference about the animal’s inner mental state.

“[The behavior] differs from strictly functional expressions of behavior structurally or temporally in at least one respect: incomplete (generally through inhibited or dropped final elements), exaggerated, awkward, precocious, or involves behavior patterns with modified form, sequencing, or targeting.”

“The behavior is performed repeatedly in a similar, but not rigidly stereotyped, form during at least a portion of the animal’s ontogeny.”

“The behavior is initiated when an animal is adequately fed, clothed, healthy, and not under stress (e.g., from physical danger, harsh weather, illness, social instability, family dysfunction), or intense competing systems (e.g., feeding, mating, competition, fear): In other words, the animal is in a ‘relaxed field.’”

In general, the characteristics that animal researchers have cited for identifying play map reasonably well onto the five characteristics listed in the previous section for human play, with the exception that animal behaviorists rarely if ever include any mention of imagination or fantasy. Researchers are understandably reluctant to suggest that animal play is accompanied by imagination in the sense that human play is. For example, there is no way to know whether two rats engaging in a play fight understand it to be a representation of a real fight. It is possible that to the rats these are entirely separate categories of behavior, such that one is in no way symbolic of the other.